Xerography is a process wherein toner is selectively transferred onto a substrate and then fused. Initially, the toner is in a sump from which it is transferred onto a photoreceptor. Usually, the photoreceptor is a drum with a photoconductive coating. Using principals of static electricity, the surface of the photoreceptor receives an electrical charge. A light beam scanned over the surface of the photoreceptor can selectively discharge the photoreceptor surface. The toner is then transferred to the photoreceptor where it sticks, due to electrostatic attraction, to those parts of the photoreceptor that have been discharged. The photoreceptor is thereby coated with patterned toner.
A substrate, such as paper, can also receive an electrical charge. The substrate can be given a larger electrical charge than the photoreceptor so that the patterned toner is transferred to the substrate when the photoreceptor contacts it. The substrate is then heated so that the patterned toner fuses to the substrate surface. Those skilled in the arts of printing, photocopying, and xerography know the details of the xerographic process, the components used in the xerographic process, and the variations in the process details and components that occur in different embodiments of xerography.
When a drum photoreceptor is used, the toner is rolled on the substrate. To faithfully transfer the patterned toner, both the substrate and the drum move. The direction that the drum moves is the process direction. The path along which the substrate travels is the process path.
A light beam scanning across the photoreceptor can discharge the electric charge on the surface. Tracing a light beam across the photoreceptor creates a line, called a scan line. Modulating the light beam during a scan can cause selective discharging along the scan line. Making many scan lines as the photoreceptor moves with respect to the scanning beam can cause selective discharging within an area.
For example, a laser printer can have a light beam that moves across the surface of the photoreceptor 6000 times per second, and a photoreceptor that moves, such as a drum rotating, at ten inches per second. The printer produces 600 scan lines per inch along the process direction because the photoreceptor is moving. If the photoreceptor is 10 inches wide, modulating the light beam so that it can change 6000 times as it sweeps the photoreceptor in the less than 1/6000 of a second it takes to move across the photoreceptor results in a 600 dot per inch resolution perpendicular to the process direction.
FIG. 4, labeled as prior art, illustrates one way to obtain a modulated light beam. A laser 401 produces a light beam 403 that passes through a modulator 402 to produce a modulated light beam 108. Those skilled in the art of optoelectronics know of many devices that can modulate light beams or laser beams.
FIG. 5, labeled as prior art, illustrates another way to obtain a modulated light beam. A laser diode 501 can produce a modulated light beam 108 directly without the need for a separate modulator such as the modulator 402 of FIG. 4.
In the example above, a laser printer produced 600 scan lines per inch at a process speed of 10 inches per second. One technique to produce a higher process speed is to sweep the laser at a high rate across the photoreceptor. Another solution is to produce many scan lines concurrently. Producing many scan lines concurrently requires many modulated light beams.
FIG. 6, labeled as prior art, illustrates one way to obtain multiple light beams. A source light beam 601 passes into a splitter 602 that splits it into numerous light beams 403. Those skilled in the art of optics know of many devices and combinations of devices for use as a splitter 602. The numerous light beams 403 can then each be modulated individually. In some applications, it can be advantageous to modulate the source light beam 601 such that it is split into numerous modulated light beams. Using numerous laser diodes or similar subassemblies can also produce numerous modulated light beams.
FIG. 7, labeled as prior art, illustrates a motor polygon assembly (MPA) 109 causing numerous modulated light beams 108 to concurrently produce multiple scan lines 111 on a substrate 110. The polygon 109 is an optical element that has many facets arranged around a rotational axis. As the polygon spins, each facet reflects the modulated light beams 108 and causes them to scan across the substrate 110 creating scan lines 111. A new set of scan lines begins as each facet starts reflecting the modulated light beams 108. Advancing the substrate along the process direction controls the locations of the new scan lines.
FIG. 8, labeled as prior art, illustrates halftone bricks. A laser printer either prints a spot of toner or it doesn't. For example, a black and white laser printer can print black dots. It cannot print gray dots. Different shades of gray are obtained by printing patterns of black dots. A light shade of gray can be produced by covering a low percentage of an area of a substrate with black dots. For example, a first halftone brick 801 can have five rows and six columns to define an area with 30 pixels. A black pixel 802 receives toner while a white pixel 803 does not. Four of the pixels are black resulting in a light shade of grey. Similarly, a second halftone brick 804 has eleven black pixels and a third halftone brick 805 has 22 black pixels.
As with any machine, a xerographic engine can exhibit irregularities. One type of irregularity is streaking parallel to the process path. There are many possible causes of parallel streaking. By definition, a streak parallel to the process path occurs at the same place along the scan lines. In other words, it has a constant position in the cross process path direction. The cross process path direction is the direction perpendicular to the process path.
As a scan line is created, the light beams are modulated on and off to produce pixels. In an effort to reduce parallel streaking, the on intensity of the light beams can also be modulated based on their cross process position. The cross process position is defined as the distance from the start of the scan of the laser. For example, a printer producing a light streak that starts one inch from the start of the scan and ends two inches from the start of the scan can modulate the profile to try to minimize the streak. In this example, the profile can modulate light beam intensity by increasing the light intensity when the cross process position of the scan line is between one inch and two inches. The light beam now has at least two modulations, an on/off modulation for pixels that are dependent on the desired printed pattern, and an on intensity modulation for cross process compensation.
A single profile can compensate for parallel streaking. However, the compensation is perfect at only one area coverages. At different area coverages, the sensitivity to the source of streaking can be different, and the light modulation will not necessarily be of the correct magnitude to compensate for the streaking. A different modulation profile is required for each area coverage. A profile that completely compensates parallel streaks for one area coverage can over compensate or under compensate for a different area coverage.
A need therefore exists for systems and methods that can compensate for parallel streaking when different area coverages are printed on a substrate. Such a goal can be accomplished by using multiple simultaneous profiles.